Wireless communications continues to grow in demand and has become an integral part of both personal and business communications. Wireless communications allow users to transmit and receive data from most anywhere using wireless networks and wireless devices such as laptops, cellular devices, iPhones®, BlackBerrys®, etc.
Wireless devices are generally configured to operate in a licensed frequency spectrum and/or an unlicensed frequency spectrum (e.g., a peer-to-peer network). The licensed spectrum includes all frequency bands that require a license to operate a wireless device. In the licensed spectrum, only the spectrum licensee can build infrastructure, and allow communications and offer services across its spectrum range. The licensed spectrum is more reliable and has less traffic congestion but generally has a narrower band when compared to the unlicensed spectrum. Hence, large amounts of data may take longer to transmit using the licensed spectrum.
The unlicensed spectrum includes all frequency bands that do not require a license to operate a wireless device. In the unlicensed spectrum, any user is free to use the frequency band for short range wireless communications. Peer-to-peer direct wireless communications is performed using the unlicensed spectrum. The unlicensed spectrum is inexpensive and has a larger band when compared to the licensed spectrum but is not controlled by any third party so can be unreliable and congested due to large amounts of data passing across these frequency bands. However, when the unlicensed spectrum is not congested, it can be useful for transferring large amount of data.
In time-slotted, synchronized peer-to-peer wireless communications, time is slotted so that each packet's transmission time is exactly one slot and all the nodes (e.g., transmitters and receivers) are synchronized so that transmissions occur within slot boundaries. A target receiver expecting to receive a signal from its corresponding transmitter can experience inter-symbol interference from other peer transmissions in at least three different ways. First, inter-symbol interference can occur when other transmitters are concurrently transmitting on the same time symbol as the one on which a target receiver is expecting to receive a signal from its corresponding transmitter. Second, inter-symbol interference can occur from transmissions from other transmitters that are transmitting on time symbols before the time symbol on which the target receiver is expecting to receive a signal from its corresponding transmitter. Third, inter-symbol interference can occur from transmissions from other transmitters that are transmitting on time symbols after the time symbol on which the target receiver is expecting to receive a signal from its corresponding transmitter.
Inter-symbol interference in the second and third ways as described above are caused by the transmit filters of the interfering transmitters. For example, most every filter whether digital or analog has a delay and a “roll-off” shape which causes the desired transmission to be spread over a longer period of time than the desired time symbol. This time-domain spillage is generally unavoidable because it allows for a sharp frequency domain response of the filter.
In time-slotted, synchronized peer-to-peer wireless communications, the inter-symbol interference resulting from the time-domain spillage can be significant because of the inherently large dynamic range of transmitted signal powers. For example, the target receiver might be in close proximity to a peer that is transmitting a signal to another peer and the power level of the transmitted signal may be 60 dB or more above the anticipated signal from the transmitter to the target receiver. Even if the transmitted signal is for a prior time symbol or a subsequent time symbol when compared to the transmission time of the anticipated signal, the time-domain spillage will smear energy into the time symbol on which the target receiver is listening for the anticipated signal from the transmitter. This results in the overall signal-to-interference-plus-noise ratio (SINR) being too low for reliable detection and decoding of the anticipated signal by the target receiver.
One way of dealing with the problem of filter energy spillage is to advance or delay the target receiver's detection and decoding window to avoid this spurious energy. However, this results in the reduction of the usable portion of the current time symbol resulting in a lower achievable signal-to-noise ratio (SNR). Another drawback is that the target receiver is unable to accommodate the various delayed transmissions occurring in the current symbol in an orthogonal frequency division multiplexing (OFDM) implementation (i.e., the cyclic prefix is effectively reduced).
Therefore, it has been recognized by those skilled in the art that a need exists for methods and apparatus for minimizing inter-symbol interference in a peer-to-peer network.